A project log for Prototype SCARA

A "demo" direct drive SCARA made out of MDF. The idea is to test my SCARA Controller.

agpcooperagp.cooper 02/26/2019 at 08:050 Comments


I wanted to get it working today. So instead of debugging my version of gCodeSender, I just hacked a copy of my gCodeFilter and used Rabbit gCodeSender.

Rabbit gCodeSender has some faults but it worked:

The Sticky Note is not very flat thus some variance in the burn. Here is a close up:

The design is about 45mm x 45mm.

Note how the lines are wavy, that is the oscillation of the SCARA arm. You can see it dies down after a few millimetres. Basically I need to add a light brake to the arm to dampen the oscillation (if I was to persist with this design).

Now the truncation of the right is by design:

To Home the SCARA I set zero for a fully outstretched or straighten arms. Pretty easy.


I did spend a few hours looking a PID to control the overshoot/undershoot.

Its feasible but you need to add optical encoders for the positional feedback, and a processor between GRBL and the A4988s.

But the purpose of this project was to prove the gCode filter works rather than the basis for a proper SCARA.

The easiest way to fix the overshoot/undershoot is to let "gear friction" dampen it.

Limit Switches and Drawing Envelope

I draw up a design for limit switches and the drawing envelop. Not much envelope:

Should have used 60mm for Arm 2 rather than 50mm.


Set the controller 1/4 stepping. 1/16 is over kill for this project. 1/4 stepping will increase torque.

Cut the tracks to the reverse Y motor phases and jumped the limit switch sensors. I hope there is no interference from the stepper motor drive cables.

I bought some micro-switches but I have not worked out how to install them. I don't have a M2 screw/bolt that is long enough. Perhaps epoxy is the way to go. Even then the "knock" bolts are not long enough. Needs more though.

I need to turn the Laser off in the initialisation string for GRBL.

Once I have tested the Homing sequence this project is done.

Interference Prevention

I had a project killed (could not get it to work) because of common power supply and electrical interference from the stepper motors. I did have some success by twisting the stepper motor cables and moving them away from the electronics. But I am still worried about the limit switches and the cables running near the stepper motor cables.

Well I found a really good discussion (by chance) on the GRBL WIKI ( I will definitely try the RC filter option.